How to Optimize PCB Design for EMC Compliance

Key Takeaways

• An explanation of EMC compliance and why it’s imperative for any board.

• Where EMC issues tend to pop up in a design.

• The best PCB design practices to employ for EMC compliance.

Overlooking EMC compliance in a PCB design can result in the anguish of days of re-design while complicating time to market and other critical product development checkpoints. The best way to reduce or eliminate EMC problems is by removing the opportunity for their manifestation in the first place. By incorporating some straightforward checks and best practices, PCBs have a greater chance of meeting expensive EMC compliance standards in a minimum number of tests.

What Is EMC Compliance?

As a PCB designer, it’s a foregone conclusion that at some point, electromagnetic compatibility (EMC) issues will rear their ugly head. Tests to establish EMC can cost tens of thousands of dollars per evaluation, and that’s assuming a board passes on its first check. EMC denotes the stability of a PCB when exposed to electromagnetic interference (EMI). Think of EMC as the counterpart to EMI: where EMI indicates a board is acting as either an unintentional signal transmitter or receiver, EMC indicates that a board is operating within acceptable limits. That doesn’t mean a board is not radiating or receiving electromagnetic energy, but that it is reasonably mitigated.

Except for making a hobby kit, PCBs must be EMC-compliant. Imagine having a washing machine that behaves erratically while making or receiving a phone call, or a microwave oven that disrupts IoT device operation. Without exaggeration, interference issues like these are very real quandaries design teams face, as certain elements on a PCB are notorious for producing or picking up electromagnetic noise.

PCBs must pass the EMC compliance test in recognized laboratories before being mass-produced. This is mandatory when designing for the consumer, medical, automotive, aviation, military, and other critical industries in most countries. To limit expenditures related to testing and mandatory redesigns, design for EMC compliance needs to be considered far before the circuit board reaches production.

Cases Undercutting PCB Design for EMC Compliance

When it comes to EMC compliance, it’s a mistake to assume that only high-speed PCB designs are a source of concern. Products like industrial air dryers can fail EMI/EMC with only a low-speed controller as the culprit. In actuality,  an improperly conditioned signal from the external power supply injected significant levels of noise. The importance of determining that all the items in the design are EMC compliant cannot be understated: failure to check for EMI/EMC compliance costs time and money and can come at a point in development where either might be in short supply. As always, the engineering creed of “trust, but verify” rings true; don’t rely solely on the information provided by the manufacturer. 

EMC compliance issues go further than a switching power supply, however. Clock signals, especially high-speed ones, are notorious for radiating significant electromagnetic energy. The same applies to digital signals that transmit varying data at high speeds.

The switching power supply can be a culprit in EMC compliance issues.

Additionally, analog signal processing can be a potential pain point for EMI/EMC. Unlike digital signals with thresholds for high and low levels, analog signals have little tolerance for error in processing. While working with analog signals, be prepared to deal with signal integrity issues, as they could be easily affected by interfering sources.

PCB Design Tips for Maximizing EMC Compliance 

Although EMC compliance has been framed thus far as passing required testing, the underlying motivation is ensuring the safety and long-term reliability of the product. Here are some tips to increase the chances of complying with EMC. 

1. Keep High-Speed Signal Traces Short and Adequately Spaced

Having a high-speed signal running across the PCB is a recipe for EMC disaster, especially if it’s in the range of hundreds of MHz and more. Keep these traces short to prevent the PCB from turning into a source of electromagnetic noise. Additionally, traces need to have enough clearance in the plane to prevent coupling; the three-width or 3W gap is a good rule of thumb.

Short traces minimizes electromagnetic radiation.

2. Separate High-Speed Signals and I/O

High-speed signals and I/O connections are bad neighbors. Keep them apart to prevent electrical noise coupling from one to another. This extends between layers, too. Be careful about routing high-speed traces beneath I/O connectors or components.

3. Take Note of the Current Return Path

Just because a circuit looks right on the schematic doesn’t mean it’ll behave so on the PCB. In actual design, the current path matters. Have a good idea of the return path for signals sensitive to EMI and ensure that it does not interfere with adjacent components. Namely, avoid routing across from non-continuous plane layers, which can cause the return current to seek its path of least impedance (like through the ground pin of a nearby component).

4. Ensure the Power Source Is Filtered

A noisy power source can disrupt the entire operation of a PCB. Regardless of using an external switching power supply or an onboard regulator, filters are a necessity. 

5. Use Ground Planes to Contain High-Speed Signals

Ground planes are a fantastic buffer in keeping electromagnetic noises out or within a PCB. Surround high-speed clock signals with ground planes or have them sandwiched in a multilayer design.

Of course, using PCB design software with the right analysis tools helps to optimize PCB design for EMC compliance before an EMC test. The SI/PI tools by Cadence are a helpful companion in maximizing compliance for EMC, especially when combined with the PCB layout functionality of Allegro PCB Designer.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.